I. Technical Field
The present invention relates generally to an electronically controlled automatic transmission capable of independently learning the fluid fill volumes of its friction elements and, more particularly, to an automatic transmission capable of learning these fill volumes after start up conditions.
II. Discussion
In the present design of automatic transmissions, a series of clutches or friction elements provide a means for application and release of separate members to and from each other during the flow of power through the transmission. These clutches thereby constitute the means by which gears within the transmission are selectively engaged or disengaged from either the engine crankshaft or the transmission case. Four speed transmissions, of the type which this invention is directed, generally include any number of elements or clutches which are applied or engaged in various combinations in relation to each of the vehicles gears.
To apply each of these clutches, an electronically controlled hydraulic actuating device such as a solenoid actuated valve is used. There is typically one valve for each clutch. These valves each control fluid flow to a respective clutch apply cavity. The flow of fluid into a clutch apply cavity results in the application or engagement of that clutch. Fluid is provided to a clutch apply cavity from the transmission's fluid pump. This pump provides the required pressurization to allow fluid flow into the clutch apply cavity. Fluid flow is enabled by the opening of the solenoid actuated valve in response to a command or control signal received by the solenoid from an electronic control system.
The electronic control system typically includes a microcomputer-based transmission control module capable of receiving input signals indicative of various vehicle operating conditions such as engine speed, torque converter turbine speed, transmission output shaft speed (vehicle speed), throttle angle position, brake application, predetermined hydraulic pressures, a driver selected gear or operating condition (PRND2L), engine coolant temperature and/or the ambient air temperature. Based on the information contained in these signals, the controller generates command or control signals for causing the actuation of each of the solenoid/actuated valves which control the application and release of fluid pressure to and from the apply cavities of the clutches or frictional units of the transmission. Accordingly, the controller is programmed to execute predetermined shift schedules stored in a memory of the controller through appropriate command signals to the solenoid-actuated valves.
Typically, transmission fluid is supplied to the clutch apply cavity in two stages. First, the fluid from the pump is supplied to the clutch element by a solenoid actuated valve in a continuous and logically full-on fashion to rapidly pre-fill and stroke the clutch apply piston through a clearance that is established when the clutch is disengaged. This rapid fill rate is maintained until the remaining piston clearance is nearly zero. Next, the solenoid actuated valve undergoes a duty cycle to cyclically allow the fluid to generate an average lower flowrate of fluid to the clutch apply cavity than was provided by the first flowrate. This lower rate causes a softer application of the respective clutch element than would otherwise be associated with the solenoid valve in its previous logically full-on state. However, application of clutch elements in this way has some drawbacks.
While the vehicle engine is running, the transmission fluid pump typically maintains fluid within the transmission fluid circuits up to a predetermined level without significant trapped air. When the vehicle engine is turned off, the pump stops pumping and the fluid level within the transmission fluid circuits can drop below the predetermined level. This fluid is replaced by air. When the engine is subsequently turned back on, the air in the clutch control circuits may not be purged until after the first application of that clutch in a shift. As a result, if the gear shifted into occurs before the air is purged in the respective applying clutch circuit, then additional fluid must replace or compress this air before clutch application can occur. If additional fluid to the respective clutch element is supplied at the lower duty cycle flowrate, then the overall time required to apply the clutch element can be significantly increased. This, in effect, creates a control time lag that can affect shift quality and clutch durability. The present invention was developed in light of these drawbacks.